This application is based on patent application No. 2000-14257 filed in Japan, the contents of which are hereby incorporated by references.
This invention relates to a shape measuring apparatus for measuring an outer surface configuration of an object by reduction scanning and specifically to a technique of correcting a relative displacement between two photo-sensors provided in a photo-sensor unit of a shape measuring apparatus to enlarge a dynamic range.
As shown in FIG. 12, there is known a method for enlarging the dynamic range of CCDs (Charge-Coupled Devices) which uses two CCDs 100, 101 and a semitransparent prism 102 having a large light amount splitting ratio (hereinafter, this method is referred to as a dual CCD system).
According to this system, the CCDs 100, 101 are so arranged as to face two mutually orthogonal emerging surfaces 102a, 102b of the semitransparent prism 102, the light amount of an incident light beam is split at a large ratio of, e.g., L1:L2=99:1 by the semitransparent prism 102 so as to focus a bright image and a dark image on the CCDs 100, 101, respectively, and the dynamic range is enlarged by combining the images separately sensed by the two CCDs 100, 101.
Since the amount of incident light is suppressed to L2/(L1+L2)=0.01 in the CCD 101, light can be sensed even if the amount of incident light exceeds the photo-sensing sensitivity of the CCD 100. Accordingly, the image sensed by the CCD 100 is used when the amount of incident light lies within the photo-sensing sensitivity of the CCD 100, whereas the two sensed images are combined to use the image sensed by the CCD 101 by adjusting the brightness by multiplying it by (L1+L2)/L2, thereby enlarging the dynamic range of the CCDs when the amount of incident light exceeds the photo-sensing sensitivity of the CCD 100.
Since the two photo-sensors (CCDs) are used in the conventional dynamic range enlarging method, photo-sensing characteristics of the two photo-sensors and the photo-sensing positions of the photo-sensors need to be adjusted. As a method for adjusting the photo-sensing positions is known the one according to which two photo-sensors are respectively provided with mechanical position adjusting mechanisms and the positions of the respective photo-sensors are adjusted using these position adjusting mechanisms. This method is a position adjusting method by hardware.
The mechanical position adjusting mechanisms are excellent in guaranteeing the positional precision of the photo-sensors by mechanical precision, but are required to have a high precision of adjustment in the order of xcexcm and need to be complicated adjusting mechanisms adjustable in directions of six axes. This brings about problems of difficult position adjustment, larger size, and high price. To this end, a method has been proposed according to which a relative displacement of two photo-sensors actually mounted is measured at the time of production, the measured displacement is stored as a correction value in a memory, an error resulting from the displacement of the photo-sensors is corrected by the correction value when light reception data sensed by the photo-sensors are processed. This method is a position adjusting method by software.
The position adjusting method by software reduces a burden caused by the mounting construction of the photo-sensors, but calculation needs to be made for correction using the correction value every time the light reception data of the photo-sensors are processed. This increases a burden on data processing performed in a CPU (Central Processing Unit).
The shape measuring apparatus for measuring an outer surface configuration of an object to be measurement or a measurement object is provided with a photo-sensor formed of a line sensor arranged in parallel to X-direction if it is assumed that a height direction (vertical direction) of the measurement object is Y-direction, a direction of a line connecting the shape measuring apparatus and the measurement object is Z-direction, and a direction normal to Y-direction and Z-direction is X-direction. The apparatus is placed at a specified height position (specified Y-coordinate position) with respect to the measurement object, light reflected by the measurement object is sensed by the photo-sensor, distances (corresponding to Z-coordinates) from the measuring apparatus (precisely from the photo-sensor) to the outer surface of the measurement object are measured for the respective pixels (for the respective X-coordinate positions). In this way, three-dimensional data (X, Y, Z) of the outer surface of the measurement object are measured at specified measurement intervals.
Since high-precision measurement is made by reduction scanning in the shape measuring apparatus, a photo-sensor having a high sensitivity and a wide dynamic range is required. However, a desired dynamic range cannot sometimes be obtained with presently commercially available photo-sensors. In such a case, a dynamic range enlarging method as described above needs to be adopted.
In the case of adopting the dual CCD system, the displacement between the two line sensors causes a problem as described above. Particularly, a displacement in a pixel aligning direction (X-direction) of the line sensor largely influences a measurement accuracy since the respective pixel positions correspond to X-coordinates of measurement points, and light reception signals at different pixel positions are combined when the light reception signals of the two line sensors A, B are switchingly combined.
FIG. 14 is a diagram showing an influence on the measurement accuracy when the two line sensors are displaced in the pixel aligning direction. In FIG. 14, a black-and-white strip CH at the uppermost stage is a test chart, and two strips A, B therebelow are line sensors. It should be noted that a1, a2, . . . a12 within the line sensor A and b1, b2, . . . b12 within the line sensor B represent pixels, which have a sensitivity characteristic as shown in FIG. 15. As shown in FIG. 14, the line sensors A, B are displaced from each other by one pixel in the pixel aligning direction. A white area (brightness level BH) and a black area (brightness level BL) of the test chart CH are sensed by the pixels a1 to a6 and the pixels a7 to a12 in the line sensor A, respectively. On the other hand, the white area and the black area are sensed by the pixels b1 to b5 and the pixels b6 to b12 in the line sensor B.
In FIG. 14, an upper graph shows the output level of the line sensor A, a middle graph shows the output level of the line sensor B, and a lower graph shows a combination of the output levels of the line sensors A and B. The amount of incident light on the line sensor B is 1/N of that on the line sensor A. In the lower graph, the output level of the line sensor B is combined with that of the line sensor A after being multiplied by N.
Since the brightness level BH of the white area exceeds a maximum output level Vmax of the line sensor A as shown in FIG. 15, the output levels of the pixels a1 to a6 of the line sensor A are saturated at the maximum output level Vmax, and the output levels of the pixels a7 to a12 are at VL corresponding to the brightness level BL of the black area. On the other hand, since the amount of incident light is gradually reduced to 1/N in the line sensor B, the output levels of the pixels b1 to b5 are at an output level VH/N corresponding to the brightness level BH/N, and the output levels of the pixels b6 to b12 are saturated at a minimum output level Vmin.
According to the dual CCD system, the output levels of the pixels a1 to a6 of the line sensor A are combined by being replaced by the output levels of the corresponding pixels b1 to b6 of the line sensor B since they are saturated. However, as shown in the lower graph, the output level corresponding to the pixel a6 is substantially missing. As a result, a plurality of light reception signals for calculating Z-coordinates (surface positions of the measurement object) are missing at the X-coordinate position corresponding to the pixel a6, and Z-coordinates at this pixel position cannot be precisely obtained.
Referring back to FIG. 13, a displacement in a direction (Z-direction) normal to the sensing surfaces of the line sensors A and B causes a difference in the amount of the received light between the line sensors A and B. Thus, even in the case that there is no displacement in the pixel aligning direction (X-direction), the output of the line sensor A and that of the line sensor B are mixed in a plurality of light reception signals for calculating Z-coordinates at a certain pixel position if the light reception signals of the line sensor A and those of the line sensor B are switchingly combined. This causes a calculation error of Z-coordinates at this pixel position.
Accordingly, if a light sensing system adopting the dynamic range enlarging method using two CCDs is used in the shape measuring apparatus, a measurement error resulting from the displacement in the pixel aligning direction (X-direction) of the line sensor and a direction of the optical axis of the light incident on the line sensor (Z-direction) need to be particularly reduced.
The conventional displacement adjusting method by hardware for mechanically adjusting the mount positions of the photo-sensors leads to a larger, more complicated and more expensive apparatus due to the adjusting mechanism and, therefore, cannot be readily adopted in the shape measuring apparatus.
If all concerning factors are collectively studied, a method for correcting a measurement error resulting from the displacement by signal processing, calculation processing or a combination of these processings when the light reception data are processed is preferable. However, no shape measuring apparatus adopting the dynamic range enlarging method using two CCDs has been conventionally known. Nor has been proposed a technique of correcting a measurement error resulting from the displacements in a pixel aligning direction of a line sensor and an optical axis direction of incident light by signal processing, calculation processing, etc.
It is an object of the present invention to provide a shape measuring apparatus which is free from the problems residing in the prior art.
According to an aspect of the present invention, a shape measuring apparatus comprises: at least two photo-sensors for converting received light to electrical light reception signals, one photo-sensor having a photo-sensing characteristic identical to another photo-sensor; a first beam splitter for splitting a light beam in a predetermined light amount ratio, and introducing split light beams to the photo-sensors, respectively; an optical system for introducing a light beam reflected from an object to be measured to the beam splitter, and having a focal point movable in relative to the object; a driver for driving the optical system to move the focal point; and a signal processing section for executing combination processing to light reception signals outputted from the photo-sensors.
The signal processing section is provided with a displacement memory for storing a relative displacement between one photo-sensor and another photo-sensor in an incident direction (or a direction perpendicular to the incident direction); a signal memory for storing light reception signals outputted from each of the photo-sensors; a corrector for correcting errors of light reception signals from one photo-sensor with respect to light reception signals from another photo-sensor in the incident direction (or perpendicular direction) based on the relative displacement stored in the displacement memory, and a signal processor for combining light reception signals from one photo-sensor with light reception signals from another photo-sensor by replacing light reception signals from a saturated part of one photo-sensor with light reception signals from a corresponding part of another photo-sensor.
According to another aspect of the present invention, a shape measuring apparatus comprises: at least two photo-sensors, each photo-sensor including a number of pixels arranged in a specified direction for converting received light to electrical light reception signals, one photo-sensor having a photo-sensing characteristic identical to another photo-sensor; a beam splitter for splitting a light beam in a predetermined light amount ratio, and introducing split light beams to the photo-sensors, respectively; an optical system for introducing a light beam reflected from an object to be measured to the beam splitter, and having a focal point movable in relative to the object; a driver for driving the optical system to move the focal point; first and second signal processing sections for executing combination processing to light reception signals outputted from the photo-sensors in their respective manners; a changer for changing over the first signal processing section and the second signal processing section; a mode setter for switchingly setting a first measurement mode and a second measurement mode; and a controller responsive to the mode setter for rendering the first signal processing section execute the combination processing when the first measurement mode is set, and rendering the second signal processing section execute the combination processing when the second measurement mode is set.
The first signal processing section includes a displacement memory for storing a relative displacement between one photo-sensor and another photo-sensor in the pixel arrangement direction (or incident direction perpendicular to the pixel arrangement direction), a signal memory for storing light reception signals outputted from each of the photo-sensors, a corrector for correcting errors of light reception signals from one photo-sensor with respect to light reception signals from another photo-sensor in the pixel arrangement direction (or the incident direction) based on the relative displacement stored in the displacement memory, and a signal processor for combining light reception signals from one photo-sensor with light reception signals from another photo-sensor by replacing light reception signals from saturated pixels of one photo-sensor with light reception signals from corresponding pixels of another photo-sensor.
The second signal processing section includes a corrector for correcting a relative displacement between one photo-sensor and another photo-sensor in the pixel arrangement direction (or the incident direction) by a pitch of pixel by delaying the sending of the reception signals from one photo-sensor with respect to the sending of the reception signals from another photo-sensor, and a signal processor for combining light reception signals from one photo-sensor with light reception signals from another photo-sensor by replacing light reception signals from saturated pixels of one photo-sensor with light reception signals from corresponding pixels of another photo-sensor.
These and other objects, features and advantages of the present invention will become more apparent upon a reading of the following detailed description and accompanying drawings.